Surface treatments such as surface hardening by heat treatment, nitriding, and the like, or shot peening, are performed on gears, shafts, or other steel products used in automobile parts and the like to improve wear-resistance, fatigue strength, etc.
Conventionally, evaluation of surface properties such as residual stress, hardness, and the like following surface treatment of these parts was done by sample destructive testing. This led to the problem that not all products could therefore not be directly tested, and because testing was destructive, tested products became unusable.
There is therefore an increasing need to develop a device capable of non-destructive testing of product surface properties. Patent Document 1, for example, discloses a non-destructive inspection device for shot-preened surfaces wherein an AC (Alternating Current) signal is input as frequency is varied to an inspection circuit furnished with a coil, disposed above a shot-peening treatment surface, and frequency response characteristics of the impedance in that test circuit are used to inspect the state of residual stress in the object under inspection.    Patent Document 1: JP-A-2008-2973